The present invention relates to speed ratio control of an automatic transmission, and more particularly, to speed ratio control when a vehicle behavior control device is operating.
A vehicle dynamics controller (VDC) is known in the art which controls yawing and rolling when a vehicle is running. The VDC increase the running stability of the vehicle by automatically adjusting the braking force of the vehicle wheels and engine output so that the yaw rate of the vehicle follows the target yaw rate decided based on the steering angle and vehicle speed. The yaw rate may also be controlled by controlling the traction force or changing the drive force distribution.
However, when the gear position of the automatic transmission is changed over when the VDC is operating, a torque fluctuation or speed fluctuation is caused by the speed change, and vehicle behavior control performance of the VDC may be impaired. A similar problem occurs if the gear position is changed over when a behavior control device other than the VDC which controls unstable behavior in the yaw direction and roll direction of the vehicle, is operating.
A control which prohibits speed change of an automatic transmission due to running conditions such as when the vehicle turns, is disclosed in JP-A-H10-89460 published by the Japanese Patent Office in 1998.
It is therefore an object of this invention to prevent loss of vehicle behavior control performance by prohibiting the speed change of an automatic transmission when a vehicle behavior control device is operating.
In order to achieve above object, the present invention provides a speed ratio control device for use with a vehicle provided with an automatic transmission and a behavior control device which controls either of yawing and rolling of the vehicle. The device comprises an actuator which changes a speed ratio of the transmission, and a microprocessor programmed to determine whether or not the vehicle behavior control device is operating, and prohibit a speed change operation by the actuator when the vehicle behavior control device is operating.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.